1. Field of the Invention
This invention relates to liquid crystal light valve (LCLV) projectors and in particular, to a two-lens optical system for use in three-color liquid-crystal light valve projectors wherein two separate optical paths are provided.
2. Description of the Prior Art
The development of the liquid crystal light valve has opened the door to substantial progress in the state of the art of high quality large screen projectors. The reflective-mode liquid crystal light valve is a thin film, multilayer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer, and a photoresponsive layer sandwiched between two transparent electrodes. A polarized projection beam is directed through the liquid crystal layer to the dielectric mirror. An input image of low intensity light, such as that generated by a cathode ray tube, is applied to the photoresponsive layer thereby switching the electric field across the electrodes from the photoresponsive layer onto the liquid crystal layer to activate the liquid crystal ("on" state). Linearly polarized projection light passing through the liquid crystal layer and reflected from the dielectric mirrors is polarization-modulated in accordance with the information incident on the photoconductor. Therefore, if a complex distribution of light, for example, a high resolution input image, but of low intensity or brightness, is focused onto the photoconductor surface, the device converts the image into a replica which can be projected with magnification to produce a high brightness image on a viewing screen. U.S. Pat. No. 4,019,807 issued to D. D. Boswell et al. on Apr. 26, 1977 disclosed such a high performance reflective mode liquid crystal light valve.
A graphics display projector using a liquid crystal light valve of the above type is described in an article entitled "Application of the Liquid Crystal Light Valve to a Large Screen Graphics Display," published in the 1979 Society for Information Display (SID), International Symposium, Digest of Technical Papers, May 1979, pp. 22-23. This display system, a type with which the present invention is particularly but not exclusively concerned, projects a large scale image having yellow-white characters on a dark blue background. The system includes a cathode ray tube (CRT) which provides input imagery; projection optics which provide the bright collimated output beam and necessary light polarization; and the liquid crystal light valve which interfaces the input and output function.
The system uses a powerful light source such as a xenon arc lamp to illuminate the liquid crystal light valve through collimating and polarizing optics. Light emitted from the xenon arc lamp is transmitted to a main polarizing prism where it is separated into S and P components where the P component passes through the prism while the S component is reflected toward the light valve. Information displayed by cathode ray tube is transferred by fiber optics to one side of the light valve which changes the polarization state from S to P. The light is then transmitted through the prism and imaged on a screen by a projector lens. In this capacity, the main prism functions as an analyzer, converting modulations of polarizaton to modulations of brightness or intensity.
The quality of the projected image is generally a function of brightness, resolution and contrast. Image quality can generally be improved by placing a prepolarizing prism in the optical path in front of the main polarizing prism. The prepolarizing prism is somewhat effective in overcoming the deficiencies in the main polarizing prism. That is, since the main polarizing prism is not 100% effective in transmitting light of one polarization and reflecting light of another, light of an undesirable polarization may reach the light valve and be modulated and reflected back through the main prism onto the projection lens. This often results in distortions of color and/or reductions in contrast and resolution.
Since the prepolarizing prism may, for reasons of cost, be of the same design as the main prism, it would typically have similar reflectance and transmittance characteristics. However, when the two prisms are used in combination, the additive effect is such as to greatly improve the quality of the projected image. The prepolarizing prism substantially removes light of one polarization from the beam which illuminates the main prism. The main prism then acts on the beam to substantially remove the residual light of the undesirable polarization state.
However, in some applications it is desirable to use a second liquid crystal light valve for enhanced information displaying capability and versatility. In this application, the use of the prepolarizing prism becomes problematic insofar as the second light valve would require light of the polarization that would otherwise be removed by the prepolarizing prism. As a result, the use of a second light valve forced a compromise in the quality of the projected image.
This problem was addressed by the Applicant and Roy Cedarstrom in a copending application entitled "Two-Color Liquid Crystal Light Valve Image Projection System with Single Prepolarizer" U.S. patent application Ser. No. 334,679, filed Dec. 28, 1981 and assigned to the assignee of this patent application. It provides color selective prepolarization of the light incident upon two or more light valves. This is accomplished by use of a prepolarizing prism which acts on light from a light source to direct light of a first polarization to a first dichroic separator and light of a second polarization to a second dichroic separator. The resulting beams are recombined in a dichroic adder prior to being applied to the main polarizing prism. The main prism directs light of a first color and first polarization state to a first light valve and light of a second color and second polarization state to a second light valve in the conventional manner.
A second copending application by the Applicant entitled "High Efficiency Optical Tank for Three Color Liquid Crystal Light Valve Image Projection With Color Selective Prepolarization," U.S. patent application Ser. No. 334,680, filed Dec. 28, 1981, and assigned to the assignee of the present invention describes a system which provides full color, high contrast image projection with an oil coupled optical arrangement. In particular, a unitary optical arrangement with four beam splitters, two filters and a beam combiner is provided. The first beam splitter separates light from a source into first and second beams with the first beam having a first color component and the second beam having second and third color components. This second beam splitter is mounted within the container in optical alignment with the first for directing light in the first beam to a first light valve. The light valve modulates the polarization of the light in the first beam and returns it to the second beam splitter where modulations of polarization are converted to modulations of intensity in the conventional manner. The second beam splitter reflects the light to a first projection lens. The third beam splitter is mounted in optical alignment with the first beam splitter. It separates the second beam into third and fourth beams having first and second polarization states, respectively. The third beam is reflected to a first filter which passes light of a second color. The fourth beam is transmitted to a second filter which passes light of a third color.
The output of each filter is a substantially monochromatic beam of a single polarization state. The filter outputs are recombined into a single beam which illuminates a fourth beam splitter. The fourth beam splitter is the main polarizing prism. It reflects light of the first polarization state to a second light valve and transmits light of the second polarization state to a third light valve. The light valves modulate the polarization state of incident light in the conventional manner and return it to the main prism where it is recombined into a single beam. This beam is directed to a second projection lens.
Although the system described in the second copending application provides many advantages, the relatively large number of optical components required makes the system expensive and relatively complex. In addition, the light loss introduced into the system due to the number of optical components utilized reduces the efficiency (amount of light collected at the light valves) of the optical system.